Atomic number
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
The number of protons in a nucleus, which defines the element.
Atomic number in plain English
The atomic number is simply how many protons (positively charged particles) sit in an atom's center, and that single count is what decides which element you're looking at — it's the atom's ID number. Every carbon atom in the universe has exactly 6 protons; change it to 7 and it's now nitrogen, a totally different element. Like a jersey number that defines the player: 6 always means carbon, no exceptions. That's why the periodic table is ordered by atomic number, counting up one proton at a time.
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